Duplex stainless clad steel plate and method of producing same

ABSTRACT

Disclosed is a duplex stainless clad steel plate in which a duplex stainless steel plate as a cladding metal is bonded or joined to one or both surfaces of a base steel plate, in which the base steel plate comprises a predetermined chemical composition such that Nb/N is 3.0 or more and Ceq is 0.35 to 0.45, and the duplex stainless steel plate comprises: a predetermined chemical composition such that PI is 34.0 to 43.0; and a microstructure containing a ferrite phase in an area fraction of 35% to 65%, and in the microstructure, an amount of precipitated Cr is 2.00% or less and an amount of precipitated Mo is 0.50% or less.

TECHNICAL FIELD

This disclosure relates to a duplex stainless clad steel plate used for,e.g., a reaction vessel of a chemical plant, and to a method ofproducing the same.

BACKGROUND

Conventionally, duplex stainless steel has been adopted under highchloride environment such as seawater or under severe corrosiveenvironment such as oil wells or gas wells. Specifically, duplexstainless steel is adopted in piping in oil wells and gas wells,structural members such as in flue gas desulfurization equipment,wastewater treatment plants, and seawater pumping power generators,paper making rolls, centrifugal separators, pumps and valves, heatexchangers, and so on. The duplex stainless steel is a stainless steelhaving a complex structure in which two phases of austenite phase andferrite phase are mixed, and has both excellent corrosion resistance andexcellent strength properties. This steel is generally known to havemost excellent corrosion resistance when the area ratio (phase fraction)of austenitic phase to ferrite phase is approximately 1:1. Therefore,the practical chemical composition of the duplex stainless steel is setsuch that the area ratio (phase fraction) of austenite phase to ferritephase is almost within this range. From this point of view, for example,SUS329J1, SUS329J3L, and SUS329J4L are standardized as steel bars andplate materials in Japanese Industrial Standards (JIS). In addition,SUS329J1FB as forged steel products and SCS10 as cast steel products arestandardized.

On the other hand, prices of alloying elements represented by Cr, Ni,and Mo, which are the main raw materials of duplex stainless steel,sometimes have soaring and great fluctuations. Accordingly, from theperspective of making use of excellent corrosion resistance of duplexstainless steel, it may be more economically feasible to use a duplexstainless steel as a clad steel having the same thickness as a solidmetal than to use as a solid metal.

A clad steel plate is a steel plate where two or more steel plateshaving different properties are bonded or joined together, such as asteel plate obtained by bonding or joining a high-alloy steel materialshowing high corrosion resistance as a cladding metal to a base steelplate made of a so-called common steel material such as carbon steel. Aclad steel plate is formed by metallurgically bonding or joining twotypes of metals having different properties, and unlike platings, thereis no concern of peeling. Further, a clad steel plate has variousproperties which cannot be achieved with a single metal or alloy.

For example, by selecting a steel material having corrosion resistanceaccording to the operating environment as a cladding metal, it ispossible to provide corrosion resistance equivalent to that of a solidmetal while suppressing the use amount of expensive alloying elements.In addition, high strength and high toughness carbon steel and low-alloysteel can be applied to the base steel plate. As such, in a clad steelplate, it is possible to provide corrosion resistance equivalent to thatof a solid metal while suppressing the use amount of expensive alloyingelements, as well as strength and toughness equivalent to those ofcarbon steel and low-alloy steel. Therefore, a clad steel plate has anadvantage that it can offer both cost advantage and functionality.

For this reason, a clad steel plate using a high-alloy steel material asa cladding metal is considered to be a very useful functional steelmaterial, and in recent years the needs thereof have been increasingmore and more in various industrial fields.

As a technique relating to such a clad steel plate, JP200061655A (PTL 1)describes “a base metal for clad steel having excellent toughness whilebeing solutionized, comprising a chemical composition containing, by wt%, C: 0.15% or less, Si: 0.5% or less, Mn: 1.5% or less, Ni: 3.0% orless, Ti: 0.008% to 0.025%, B: 0.0004% to 0.0020%, and N: 0.006% to0.015%, with the balance being Fe and inevitable impurities (claim 1)”.

CITATION LIST Patent Literature

-   PTL 1: JP200061655A

SUMMARY Technical Problem

In particular, SUS316L clad steel has been used as a material forreaction containers of chemical plants. In recent years, there is anincreasing demand for alternative duplex stainless clad steel such asSUS329J3L clad steel which is more corrosion resistant than SUS316L cladsteel. However, our study revealed that the conventional duplexstainless clad steel plates are insufficient in terms of corrosionresistance when used as cladding metal. Since PTL 1 describes a claddingmetal of clad steel only as having appropriate material properties, itis not possible to identify the characteristics of the clad steel as awhole obtained by combining the cladding metal and the base metal. Forthis reason, sufficient corrosion resistance can not be obtained in theclad steel using the base metal described in PTL 1.

Further, in recent years, the use of clad steel in low temperatureenvironment such as cold districts is increasing, and higher base metalstrength and toughness are required for clad steel.

It would thus be helpful to provide a duplex stainless clad steel plateexcellent in the corrosion resistance of a cladding metal and in thestrength and toughness of a base metal, and a method of producing thesame.

Solution to Problem

As a result of intensive studies made to solve the above problems, wediscovered the following.

In duplex stainless steel used for duplex stainless clad steel, variousproperties may change in accordance with a change in metallic structuredue to thermal influence. For example, the ferrite phase increases in ahigh temperature range of the melting point to 1200° C. In anintermediate temperature range of 600° C. to 900° C., heterogeneousphases, e.g., intermetallic compounds such as sigma phase andcarbonitrides are precipitated. In a low temperature range of 450° C. to500° C., a reaction considered as decomposition of the ferrite phaseoccurs. As described above, the metallic structure changes in respectivetemperature ranges, and corrosion resistance and strength propertieschange accordingly. Among the above-mentioned structural changes,precipitation of intermetallic compounds such as sigma phase, carbidessuch as Cr₂₃C₆, and nitrides such as Cr₂N is a problem. When at leastone of sigma phase, a carbide, a nitride, or a carbonitride isprecipitated, a depletion layer of a corrosion-resistant element such asCr or Mo is formed around the precipitate, and the corrosion resistanceis remarkably deteriorated.

In order to improve the corrosion resistance in duplex stainless steel,it is conceivable to alter the alloy components. For example, if the Crcontent is decreased, sigma phase precipitation is less likely to occur.This is because sigma phase is basically structured such that Fe:Cr=1:1.Likewise, sigma phase precipitation can be delayed by reducing the Mocontent. However, if the Cr and Mo contents are reduced, the corrosionresistance of the matrix phase of the cladding metal is adverselyaffected. That is, the delay in the sigma phase precipitation in thismethod would cause deterioration in corrosion resistance as a whole. Itis thus not preferable to reduce the content of Cr or Mounconditionally. Also, if the C content is decreased, carbides are lesslikely to precipitate. However, an extremely lowered C content increasesthe smelting load, resulting in increased manufacturing costs.

As described above, there has not been established any method that canincrease the corrosion resistance of the cladding metal as a whole bypreventing precipitation of sigma phase, carbides, and the like in theduplex stainless steel in other ways than improving alloy components. Inparticular, in the case of producing a clad steel, the restriction ofmaintenance of the mechanical properties of the base metal makes itdifficult to carry out solution treatment to dissolve sigma phase,carbides, and the like, and the problem of deterioration of thecorrosion resistance of a duplex stainless steel of a cladding metalresulting from precipitation of sigma phase, carbides, and the likeremains to be solved.

We investigated the relationship between precipitates and corrosionresistance using various test materials made of duplex stainless steel.As a result, corrosion resistance deterioration was found to be causedby precipitates such as sigma phase, carbides, nitrides, andcarbonitrides. Further, with respect to a duplex stainless steel of acladding metal, it was discovered that there is a correlation betweenthe amounts of Cr and Mo contained in these precipitates (that is, theamounts of Cr and Mo present as precipitates) and corrosion resistance.We also found the production conditions for reducing the amount of Crpresent as a precipitate even in the case where the Cr content of theduplex stainless steel as the cladding metal is relatively high.

It was further discovered that the strength and toughness of the basemetal are improved by limiting the index Ceq defined by a predeterminedrelational expression and Nb/N to the predetermined ranges with respectto the chemical composition of the base metal. The present disclosurewas completed based on these discoveries, and primary features thereofare as described below.

[1] A duplex stainless clad steel plate in which a duplex stainlesssteel plate as a cladding metal is bonded or joined to one or bothsurfaces of a base steel plate, wherein the base steel plate comprises afirst chemical composition containing (consisting of), in mass %, C:0.06% to 0.25%, Si: 0.05% to 0.50%, Mn: 0.70% to 1.60%, P: 0.030% orless, S: 0.010% or less, Al: 0.005% to 0.100%, Mo: 0.01% to 0.15%, Nb:0.010% to 0.040%, Ti: less than 0.005%, and N: 0.0010% to 0.0100%, withthe balance being Fe and inevitable impurities, in a range such thatNb/N is 3.0 or more and Ceq represented by the following Expression (1)is 0.35 to 0.45:Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5  (1),where the symbol of each element indicates the content in mass % of theelement, and any element not contained is calculated as 0, the duplexstainless steel plate comprises: a second chemical compositioncontaining (consisting of), in mass %, C: 0.030% or less, Si: 1.00% orless, Mn: 2.00% or less, P: 0.050% or less, S: 0.0100% or less, Ni:5.00% to 8.00%, Cr: more than 24.0% and not more than 28.0%, Mo: 2.5% to4.0%, and N: 0.08% to 0.30%, with the balance being Fe and inevitableimpurities, in a range such that PI defined by the following Expression(2) is 34.0 to 43.0:PI=Cr+3.3Mo+16N  (2),where the symbol of each element indicates the content in mass % of theelement; and a microstructure containing a ferrite phase in an areafraction of 35% to 65%, and in the microstructure, an amount ofprecipitated Cr is 2.00% or less and an amount of precipitated Mo is0.50% or less.

[2] The duplex stainless clad steel plate according to [1], wherein thesecond chemical composition of the duplex stainless steel plate furthercontains, in mass %, at least one selected from the group consisting ofCu: 1.50% or less, W: 1.50% or less, Co: 1.50% or less, Ti: 0.25% orless, and Nb: 0.25% or less.

[3] The duplex stainless clad steel plate according to [1] or [2],wherein the first chemical composition of the base steel plate furthercontains, in mass %, at least one selected from the group consisting ofCu: 0.50% or less, Ni: 0.50% or less, Cr: 0.40% or less, and V: 0.050%or less.

[4] A method of producing a duplex stainless clad steel plate in which aduplex stainless steel plate as a cladding metal is bonded or joined toone or both surfaces of a base steel plate, the method comprising:preparing a clad slab by stacking a first blank plate to be the basesteel plate and a second blank plate to be the duplex stainless steelplate as the cladding metal in a layered manner, the first blank platecomprising the first chemical composition as recited in [1] or [3], andthe second blank plate comprising the second chemical composition asrecited in [1] or [2] and a microstructure containing a ferrite phase inan area fraction of 35% to 65%; heating the clad slab to 1050° C. to1250° C.; then hot rolling the clad slab with a rolling reduction ratioof 2.0 or more to obtain a rolled clad body in which the base steelplate and the duplex stainless steel plate are bonded or joinedtogether; allowing the rolled clad body to naturally cool; thenreheating the rolled clad body to 1000° C. to 1100° C.; then cooling therolled clad body such that the duplex stainless steel plate is cooled ata cooling rate of 0.8° C./s or higher and the base steel plate is cooledat a cooling rate of 1.0° C./s or higher; and then tempering the rolledclad body at 700° C. or lower.

Advantageous Effect

According to the method of producing a duplex stainless clad steel platedisclosed herein, it is possible to produce a duplex stainless cladsteel plate excellent in the corrosion resistance of the cladding metaland in the strength and toughness of the base metal. The duplexstainless clad steel plate disclosed herein is excellent in all of thecorrosion resistance of the cladding metal and the strength andtoughness of the base metal.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross-sectional view of a clad slab.

DETAILED DESCRIPTION

(Duplex Stainless Clad Steel Plate)

One of the embodiments of the present disclosure relates to a duplexstainless clad steel plate in which a duplex stainless steel plate as acladding metal is bonded or joined to one or both surfaces of a basesteel plate. The thickness of the duplex stainless clad steel plate isnot particularly limited, yet is preferably 6 mm to 45 mm. Also, thethicknesses of the base steel plate and the cladding metal arepreferably about 5 mm to 40 mm and 1 mm to 5 mm, respectively. In thisembodiment, it is possible to achieve both improvement of pittingcorrosion resistance and improvement of strength and toughness by usinga duplex stainless clad steel in which a specific base metal is combinedwith a specific cladding metal.

Hereinafter, the chemical compositions of the base steel plate and thecladding metal which are components of the duplex stainless clad steelwill be described in detail. Hereinafter, the unit “%” relating to thecontent of elements in the chemical composition refers to “mass %”unless specified otherwise.

[Chemical Composition of Base Steel Plate]

By using a low-carbon steel having the following chemical compositionfor the base steel plate, it is possible to provide a duplex stainlessclad steel excellent in mechanical properties such as strength andtoughness.

C: 0.06% to 0.25%

C is an element for increasing the strength of steel, and when it iscontained in an amount of 0.06% or more, sufficient strength isexhibited. Therefore, the C content is 0.06% or more, and preferably0.08% or more. However, when the C content exceeds 0.25%, weldabilityand toughness are deteriorated. Therefore, the C content is set to 0.25%or less, and preferably 0.20% or less.

Si: 0.05% to 0.50%

Si is effective for deoxidation and is contained in an amount of 0.05%or more in order to improve the strength of steel. Also, Si is anelement inevitably entering into steel from raw materials such as ironore, and keeping the Si content below 0.05% leads to an increase in costin the steelmaking process. Therefore, the Si content is 0.05% or more,and preferably 0.10% or more. However, a Si content of greater than0.50% can lead to deterioration in the surface characteristics andtoughness of the steel. Therefore, the Si content is 0.50% or less, andpreferably 0.45% or less.

Mn: 0.70% to 1.60%

Mn is an element that increases the strength of steel. This effect isexhibited when Mn is contained in an amount of 0.70% or more. Therefore,the Mn content is 0.70% or more, and preferably 1.00% or more. However,when the Mn content exceeds 1.60%, the weldability is impaired and thealloy cost also increases. Therefore, the Mn content is 1.60% or less.

P: 0.030% or Less

P is an inevitable impurity in the steel, and when the P content exceeds0.030%, the toughness deteriorates. Therefore, the P content is 0.030%or less, preferably 0.020% or less, and more preferably 0.015% or less.However, from the viewpoint of the dephosphorization cost, the P contentis preferably 0.0001% or more.

S: 0.010% or Less

As in P, S is also an inevitable impurity in steel. When the S contentexceeds 0.010%, the toughness deteriorates. Therefore, the S content is0.010% or less, preferably 0.005% or less, and more preferably 0.003% orless. However, from the viewpoint of desulfurization cost, the S contentis preferably 0.0001% or more, and more preferably 0.0003% or more.

Al: 0.005% to 0.100%

Al is added as a deoxidizing agent. Al exhibits a deoxidizing effectwhen contained in an amount of 0.005% or more. Therefore, the Al contentis 0.005% or more, and preferably 0.010% or more. However, an Al contentof more than 0.100% can lead to deterioration in the toughness of thewelded portion. Therefore, the Al content is 0.100% or less, andpreferably 0.070% or less.

Mo: 0.01% to 0.15%

Mo is an element for increasing the hardenability of steel, and improvesthe strength and toughness of the steel after rolling. This effect isexhibited when the Mo content is 0.01% or more. Therefore, the Mocontent is 0.01% or more, and preferably 0.05% or more. However, a Mocontent of more than 0.15% can lead to deterioration in weldability.Therefore, the Mo content is 0.15% or less.

Nb: 0.010% to 0.040%

Nb precipitates as a Nb nitride, having the effect of suppressing thecoarsening of austenite grains and increasing the strength and toughnessof steel. Nb also has the effect of expanding the recrystallizationtemperature range to low temperature range during rolling in theaustenite region, thereby enabling refinement of grains and increasingthe toughness. These effects are obtained by containing 0.010% or more.Therefore, the Nb content is 0.010% or more, preferably 0.013% or more,and more preferably 0.015% or more. However, when the Nb content exceeds0.040%, coarse Nb nitride is formed, deteriorating the toughness.Therefore, the Nb content is 0.040% or less, preferably 0.035% or less,and more preferably 0.030% or less. Further, by setting thebelow-described ratio with respect to the nitrogen atom to be not lessthan a predetermined value, the effect of suppressing coarsening ofaustenite grains can be further enhanced.

Ti: less than 0.005%

In the case of this embodiment containing Nb as an essential element, Tiforms a composite carbide and/or composite nitride with Nb. With the Nbcontent specified in this embodiment, it was confirmed that coarsecomposite carbides and/or composite nitrides of Ti and Nb are formedwhen the Ti content is 0.005% or more, causing the toughness todeteriorate. Therefore, the Ti content is less than 0.005%, preferably0.003% or less, and more preferably 0.001% or less. The Ti content ispreferably reduced as much as possible, yet may be, for example, 0.0001%or more, or 0.0003% or more.

N: 0.0010% to 0.0100%

N is an element indispensable for formation of Nb nitride, and a Nbnitride is formed when the N content is 0.0010% or more. Therefore, theN content is 0.0010% or more, preferably 0.0020% or more, and morepreferably 0.0025% or more. However, a N content of more than 0.0100%can lead to deterioration in weldability and toughness. Therefore, the Ncontent is 0.0100% or less, preferably 0.0070% or less, and morepreferably 0.0050% or less. Further, by setting the below-describedratio with respect to Nb to be not less than a predetermined value, itis considered that the effect of suppressing coarsening of γ grains(austenite grains) can be further enhanced.

Nb/N: 3.0 or More

When Nb/N is 3.0 or more, the precipitation of Nb nitride and the effectof solute Nb are fully developed. However, when Nb/N is less than 3.0,the toughness remarkably deteriorates due to the presence of solute N inthe steel. Therefore, Nb/N is set to 3.0 or more, and preferably 3.5 ormore. Further, Nb/N can be 20.0 or less.

Ceq: 0.35 to 0.45

Ceq is an index of the hardenability of steel and is represented by:Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5  (1),where the symbol of each element indicates the content in mass % of theelement, and any element not contained is calculated as 0.

By setting Ceq to 0.35 or more, it is possible to secure sufficienthardenability and to provide the steel with good strength and toughness.Therefore, Ceq is set to 0.35 or more, and preferably 0.38 or more.However, when Ceq exceeds 0.45, the weldability is impaired. Therefore,Ceq is set to 0.45 or less.

In addition to the above basic components, the steel may furthercontain, as optional component(s), at least one selected from the groupconsisting of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.40% or less,and V: 0.050% or less.

Cu: 0.50% or Less

Cu is an element for increasing the hardenability of steel, and improvesthe strength and toughness of the steel after rolling. This effect isexerted when the Cu content is 0.01% or more. Therefore, in the case ofexpecting improvement of hardenability being accomplished by adding Cu,it is preferable to set the Cu content to 0.01% or more, and morepreferably to 0.05% or more. However, a Cu content of more than 0.50%can lead to deterioration in weldability and toughness. Therefore, whenCu is added, the Cu content is set to 0.50% or less.

Ni: 0.50% or Less

Ni is an element which improves the hardenability of steel and isparticularly effective for increasing toughness. This effect is exertedwhen the Ni content is 0.01% or more. Therefore, in the case ofexpecting improvement of hardenability being accomplished by adding Ni,it is preferable to set the Ni content to 0.01% or more, and morepreferably to 0.05% or more. However, when the Ni content exceeds 0.50%,the weldability is impaired and the alloy cost also increases.Therefore, when Ni is added, the Ni content is set to 0.50% or less.

Cr: 0.40% or Less

As with Cu, Cr is an element for improving the hardenability of steel,and improves the strength and toughness of the steel after rolling. Thiseffect is exerted when the Cr content is 0.01% or more. Therefore, inthe case of expecting this effect being accomplished by adding Cr, it ispreferable to set the Cr content to 0.01% or more, and more preferablyto 0.05% or more. However, a Cr content of more than 0.40% can lead todeterioration in weldability and toughness. Therefore, when Cr is added,the Cr content is set to 0.40% or less.

V: 0.050% or Less

V is an element that improves the strength of steel by formingcarbonitride. This effect is exerted when the V content is 0.001% ormore. Therefore, in the case of expecting this effect being accomplishedby adding V, it is preferable to set the V content to 0.001% or more,and more preferably to 0.005% or more. However, when the V contentexceeds 0.050%, the toughness deteriorates. Therefore, when V is added,the V content is set to 0.050% or less.

The balance other than the above is Fe and inevitable impurities. It isnoted that the base metal undergoes no remarkable change in itsproperties if its chemical composition contains at least one selectedfrom the group consisting of Ca: 0.010% or less, B: 0.0050% or less, Sn:0.050% or less, Sb: 0.050% or less, Zr: 0.050% or less, W: 0.050% orless, Co: 0.050% or less, Mg: 0.020% or less, REM: 0.010% or less, andO: 0.0100% or less in this range.

[Chemical Composition of Cladding Metal]

Next, the chemical composition of the duplex stainless steel plate as acladding metal will be described.

C: 0.030% or Less

C is one of the elements inevitably present in the steel material. Whenthe C content exceeds 0.030%, precipitation of carbides occursconspicuously, causing corrosion resistance to deteriorate. Therefore,the C content is 0.030% or less, preferably 0.025% or less, and morepreferably 0.020% or less. The C content is preferably 0.001% or morefrom the viewpoint of production cost.

Si: 1.00% or Less

Si is an element which remarkably promotes precipitation ofintermetallic compounds such as sigma phase. To suppress precipitationof intermetallic compounds such as sigma phase, the Si content needs tobe 1.00% or less. Therefore, the Si content is 1.00% or less, preferably0.50% or less, and more preferably 0.40% or less. However, from theviewpoint of production cost, the Si content is preferably 0.01% ormore.

Mn: 2.00% or Less

Mn is an element useful for deoxidation, and is preferably contained inan amount of 0.01% or more. On the other hand, when the Mn contentexceeds 2.00%, MnS is formed to degrade corrosion resistance. Therefore,the Mn content is 2.00% or less, preferably 1.70% or less, morepreferably 1.50% or less, and even more preferably 1.00% or less.

P: 0.050% or Less

A P content of more than 0.050% can lead to deterioration in toughnessand corrosion resistance. Therefore, the P content is 0.050% or less,preferably 0.040% or less, and more preferably 0.020% or less. However,from the viewpoint of the dephosphorization cost, the P content ispreferably 0.0001% or more.

S: 0.0100% or Less

A S content of more than 0.0100% can lead to deterioration in hotworkability and corrosion resistance. Therefore, the S content is0.0100% or less, preferably 0.0050% or less, and more preferably 0.0020%or less. However, from the viewpoint of desulfurization cost, the Scontent is preferably 0.0001% or more.

Ni: 5.00% to 8.00%

Ni is an essential element for stabilizing the austenite phase which isone phase of the duplex stainless steel. This effect is exerted when theNi content is 5.00% or more. Therefore, the Ni content is 5.00% or more,preferably 5.50% or more, more preferably 5.70% or more, and even morepreferably 6.00% or more. However, since Ni is an expensive metal, whenit is contained in a large amount, the alloy cost itself is increased.Accordingly, the Ni content is 8.00% or less, preferably 7.50% or less,and more preferably 7.00% or less. As described above, the duplexstainless steel has excellent corrosion resistance when the ratio ofaustenite phase to ferrite phase is 35:65 to 65:35, preferablyapproximately 1:1. Therefore, in order to satisfy this phase ratio, theNi content is set to 5.00% to 8.00%.

Cr: More Than 24.0% and Not More Than 28.0%

Cr is an essential element for ensuring the corrosion resistance of thealloy and for stabilizing the ferrite phase which is the other phase ofthe duplex stainless steel. To obtain this effect, the Cr content needsto be more than 24.0%. Therefore, the Cr content is more than 24.0%.However, when the Cr content exceeds 28.0%, sigma phase precipitation ispromoted, causing an adverse effect on ductility and toughness.Therefore, the Cr content is not more than 28.0%, preferably not morethan 27.0%, and more preferably not more than 26.0%. As described above,the duplex stainless steel has excellent corrosion resistance when theratio of austenite phase to ferrite phase is 35:65 to 65:35, preferablyapproximately 1:1. Therefore, in order to satisfy this phase ratio, theCr content is set to more than 24.0% and not more than 28.0%.

Mo: 2.5% to 4.0%

Mo is also important as an element for improving the corrosionresistance of the alloy. To obtain this effect, the Mo content needs tobe 2.5% or more. Therefore, the Mo content is 2.5% or more, andpreferably 3.0% or more. However, when the Mo content exceeds 4.0%,sigma phase precipitation is remarkably promoted, causing an adverseeffect on ductility and toughness. Therefore, the Mo content is 4.0% orless. As described above, the duplex stainless steel has excellentcorrosion resistance when the ratio of austenite phase to ferrite phaseis 35:65 to 65:35, preferably approximately 1:1. Therefore, in order tosatisfy this phase ratio, the Mo content is set to 2.5% to 4.0%.

N: 0.08% to 0.30%

N is important as an element for increasing the corrosion resistance ofthe alloy and is also effective for increasing the strength. To obtainthis effect, the N content needs to be 0.08% or more. Therefore, the Ncontent is 0.08% or more, preferably 0.10% or more, and more preferably0.14% or more. However, when the N content exceeds 0.30%, precipitationof nitrides such as Cr₂N is promoted, causing an adverse effect oncorrosion resistance. Therefore, the N content is 0.30% or less, andpreferably 0.25% or less. As described above, the duplex stainless steelhas excellent corrosion resistance when the ratio of austenite phase toferrite phase is 35:65 to 65:35, preferably approximately 1:1.Therefore, in order to satisfy this phase ratio, the N content is set to0.08% to 0.30%.

PI: 34.0 to 43.0

PI is a pitting index (pitting corrosion resistance index) defined by:PI=Cr+3.3Mo+16N  (2),where the symbol of each element indicates the content in mass % of theelement.

The higher the PI value, the better the pitting corrosion resistance is,and from the viewpoint of obtaining sufficient pitting corrosionresistance, the PI value is set to 34.0 or more, and preferably 35.0 ormore in the present disclosure. However, when the PI value exceeds 43.0,the risk of precipitation of at least one of sigma phase, carbides,nitrides, or carbonitrides increases and the alloy cost also increases.Therefore, the PI value is set to 43.0 or less, and preferably 41.0 orless.

The PI value calculated by Expression (1) is an index value of thepitting corrosion resistance of the solution-treated material in whichthe fractions of sigma phase, carbides, and nitrides are 0%. On theother hand, when precipitates such as sigma phase, carbides, nitrides,and carbonitrides are formed, the pitting corrosion resistance isdetermined by the balance between the PI value and the amounts of Cr andMo contained in these precipitates. The higher the PI value, the betterthe pitting corrosion resistance of the base metal. However, when the PIvalue is high, the content of Cr, Mo, or N naturally increases, and suchprecipitates tend to form. Then, the amount of precipitated Cr or theamount of precipitated Mo, which will be described later, increases,resulting in a decrease in pitting corrosion resistance. Therefore, inthe present disclosure, the PI value ranges from 34.0 to 43.0.

In addition to the above basic components, the steel may furthercontain, as optional component(s), at least one selected from the groupconsisting of Cu: 1.50% or less, W: 1.50% or less, Co: 1.50% or less,Ti: 0.25% or less, and Nb: 0.25% or less.

Cu: 1.50% or Less

Cu is an element for increasing corrosion resistance. This effect isexerted when the Cu content is 0.01% or more. Accordingly, in the caseof increasing corrosion resistance by adding Cu, it is preferable to setthe Cu content to 0.01% or more. However, when the Cu content exceeds1.50%, hot workability is remarkably deteriorated. Therefore, when Cu isadded, the Cu content is set to 1.50% or less, and preferably 1.00% orless.

W: 1.50% or Less

W is an element for increasing the corrosion resistance of the alloy.This effect is exerted when the W content is 0.01% or more. Therefore,in the case of increasing corrosion resistance by adding W, it ispreferable to set the W content to 0.01% or more. However, when the Wcontent exceeds 1.50%, sigma phase precipitation is promoted. Therefore,when W is added, the W content is set to 1.50% or less, and preferably1.00% or less.

Co: 1.50% or Less

Co is also an element that improves corrosion resistance. This effect isexerted when the Co content is 0.01% or more. Therefore, in the case ofincreasing corrosion resistance by adding Co, it is preferable to setthe Co content to 0.01% or more. However, when the Co content exceeds1.50%, the alloy cost increases. Therefore, when Co is added, the Cocontent is set to 1.50% or less, and preferably 1.00% or less.

Ti: 0.25% or Less

Ti has a property of being easily bonded to C, and is capable of, whencontained in the alloy, delaying precipitation of carbides such asCr₂₃C₆ which is harmful to corrosion resistance. This effect is exertedwhen the Ti content is 0.01% or more. Therefore, in the case ofexpecting this effect being accomplished by adding Ti, it is preferableto set the Ti content to 0.01% or more. However, adding Ti beyond 0.25%fails to further increase this effect, but instead the alloy costincreases. Therefore, when Ti is added, the Ti content is set to 0.25%or less, and preferably 0.20% or less.

Nb: 0.25% or Less

As with Ti, Nb also has a property of being easily bonded to C, and iscapable of, when contained in the alloy, delaying precipitation ofcarbides such as Cr₂₃C₆ which is harmful to corrosion resistance. Thiseffect is exerted when the Nb content is 0.01% or more. Therefore, inthe case of expecting this effect being accomplished by adding Nb, it ispreferable to set the Nb content to 0.01% or more. However, adding Nbbeyond 0.25% fails to further increase this effect, but instead thealloy cost increases. Therefore, when Nb is added, the Nb content is setto 0.25% or less, and preferably 0.20% or less.

The balance other than the above is Fe and inevitable impurities. It isnoted that the cladding metal undergoes no remarkable change in itsproperties if its chemical composition contains at least one selectedfrom the group consisting of Al: 0.05% or less, V: 0.2% or less, Ca:0.02% or less, B: 0.01% or less, O: 0.02% or less, Sn: 0.2% or less, Sb:0.2% or less, Zr: 0.2% or less, Mg: 0.02% or less, REM: 0.2% or less inthis range.

[Microstructure of Cladding Metal]

Area Fraction of Ferrite Phase: 35% to 65%

As used herein, the “area fraction of ferrite phase” refers either to avalue calculated by the following Expressions (3) to (5) and estimatedfrom the chemical composition of the duplex stainless steel plate, or toa value calculated from a microscope image described later.area fraction of ferrite phase (%)=4.01Creq−5.6Nieq−4.13  (3),Creq=Cr+1.73Si+0.88Mo  (4), andNieq=Ni+24.55C+21.75N+0.4Cu  (5).In Expressions (4) and (5), the symbol of each element indicates thecontent in mass % of the element, and any element not contained iscalculated as 0.

As described above, it is found that the duplex stainless steel plate asthe cladding metal exhibits corrosion resistance when the phase fractionof ferrite phase to austenite phase is in the range of 35:65 to 65:35.When the value of area fraction of ferrite phase is 35% to 65%, thephase fraction of ferrite phase and austenite phase is approximately35:65 to 65:35, and excellent corrosion resistance is exerted. From thisviewpoint, the area fraction of ferrite phase is 35% or more, preferably40% or more, and more preferably 45% or more, and is 65% or less,preferably 60% or less, and more preferably 55% or less.

The area fraction of ferrite phase can be calculated by any known methodother than Expression (3), yet, for example, it may be determined by thefollowing method. For ferrite phase, austenite phase, and precipitates(including sigma phase, carbides, nitrides, and carbonitrides),respective area fractions can be calculated by subjecting the duplexstainless steel plate to electrolytic etching and processing the colormicrographs taken by an optical microscope with image processingsoftware. It is confirmed that values calculated by this method arecorrelated with those calculated according to Expression (3). It isnoted that the area fraction is 100% in total of ferrite phase+austenitephase+precipitates (including sigma phase, carbides, nitrides, andcarbonitrides), and in the case where precipitates are not present(i.e., zero), ferrite phase+austenite phase equals 100%.

[Amount of Precipitated Cr and Amount of Precipitated Mo]

When the microstructure of the cladding metal contains Cr-basedprecipitates and Mo-based precipitates at least in predeterminedamounts, corrosion resistance deteriorates. Therefore, in the presentdisclosure, it is important that the amount of precipitated Cr and theamount of precipitated Mo (i.e., the amounts of Cr and Mo existing asprecipitates) are not greater than a predetermined amount. As usedherein, the term “precipitate” refers to one or more selected from thegroup consisting of sigma phase, carbide, nitride, and carbonitrideexisting in the microstructure of the cladding metal.

Cr and Mo are generally and widely known as elements forming a passivefilm. When Cr and/or Mo in the matrix gather in a precipitate, theconcentration of Cr and/or Mo in the periphery of the precipitatedecreases, and in a corrosive environment, this low-Cr and/or low-Moregion is preferentially corroded. This phenomenon is calledsensitization. When the amount of precipitated Cr exceeds 2.00% by mass,sensitization progresses and corrosion resistance deteriorates.Therefore, the amount of precipitated Cr is set to 2.00% or less.Similarly, when the amount of precipitated Mo exceeds 0.50% by mass,sensitization progresses and corrosion resistance deteriorates.Therefore, the amount of precipitated Mo is set to 0.50% or less. Theamount of precipitated Cr and the amount of precipitated Mo can be 0.00%or more, respectively.

(Method of Producing Duplex Stainless Clad Steel Plate)

A method of producing a duplex stainless clad steel plate according toone of the embodiments of the present disclosure will be describedbelow. A first blank plate to be the base steel plate can be prepared bya steelmaking process so as to have the above-described chemicalcomposition of the base steel plate, and may be produced in accordancewith conventional methods. A second blank plate to be the duplexstainless steel plate as the cladding metal can be prepared by asteelmaking process so as to have the above-described chemicalcomposition of the cladding metal, and may be produced in accordancewith conventional methods. The first and second blank plates are piledon top of one another in a layered manner to assemble a clad slab. Forexample, as illustrated in FIG. 1 , a clad slab 10 can be formed bypiling one stacked set of a first blank plate 1 and a second blank plate2 on another in a layered manner such that the second blank plates faceeach other. At this point, a separating agent 3 can be applied betweenthe second blank plates 2. The separating agent 3 is not particularlylimited, yet is preferably a relatively inexpensive one with sufficientseparatability, such as Al₂O3. In FIG. 1 , reference numeral 4 denotes aspacer, and reference numeral 5 denotes a welded portion. Inconsideration of warpage during cooling, it is desirable that the firstblank plates are equal in thickness to each other, and so are the secondblank plates. Of course, there is no need to limit to the assemblymethod as illustrated in FIG. 1 .

The clad slab thus obtained is heated and further subjected to hotrolling to obtain a rolled clad body in which the base steel plate andthe duplex stainless steel plate are bonded or joined together.

Heating Temperature: 1050° C. to 1250° C.

The heating temperature is set to 1050° C. or higher in order to ensurethe bonding or joining property between the base steel plate and thecladding metal and the toughness of the base steel plate. In the case ofa heating temperature below 1050° C., the rolling amount in the hightemperature region can not be sufficiently secured and the bonding orjoining property deteriorates. Therefore, the heating temperature is setto 1050° C. or higher, and preferably 1100° C. or higher. On the otherhand, when the heating temperature exceeds 1250° C., crystal grainsremarkably coarsen, and the toughness of the base steel platedeteriorates. Therefore, the heating temperature is 1250° C. or lower.

Rolling Reduction Ratio: 2.0 or More

The term “rolling reduction ratio” refers to “thickness of a clad slabbefore rolling/thickness of a rolled clad body after rolling”. When theclad slab is roll-reduced at high temperature, metals gain bonding forcesuch that a good bonding or joining property is obtained. The rollingreduction ratio is set to 2.0 or more, and preferably 3.0 or more. Thissetup can provide a good bonding or joining property. In addition, thecrystal grains of the base steel plate are refined and the toughness ofthe base steel plate is improved. The rolling reduction ratio may be20.0 or less.

Then, after being allowed to naturally cool in the atmosphere or thelike, the rolled clad body is reheated to 1000° C. to 1100° C.

As used herein, the phrase “allowed to naturally cool” means that therolled clad body is exposed to the atmosphere without forcible coolingby water injection or the like, and is subjected to air cooling ratherthan active cooling. As used herein, the term “active cooling” means “toactively cool with a gas, a liquid, or a mixture thereof”. In thisembodiment, from the viewpoint of increasing corrosion resistance,strength, or toughness, it is preferable not to perform active coolingduring reheating after allowing the rolled clad body to naturally cool.It is preferable to set the cooling stop temperature in the naturalcooling to 400° C. or lower.

Reheating Temperature: 1000° C. to 1100° C.

Reheating is performed after hot rolling in order to ensure thecorrosion resistance of the cladding metal. By performing reheatingafter hot rolling, it is possible to redissolve the precipitates andensure the corrosion resistance of the cladding metal. When thereheating temperature is below 1000° C., precipitation of sigma phaseand/or carbonitrides in the duplex stainless steel becomes significantand corrosion resistance deteriorates. Therefore, the reheatingtemperature is set to 1000° C. or higher. On the other hand, if thereheating temperature exceeds 1100° C., the crystal grains of the basesteel plate coarsen, leading to a remarkable deterioration in thetoughness of the base steel plate. Therefore, the reheating temperatureis set to 1100° C. or lower, and preferably 1050° C. or lower.

The rolled clad body after reheating is cooled. At this time, the duplexstainless steel plate of the cladding metal and the base steel plate arecooled at different cooling rates.

Cooling Rate of Cladding Metal After Reheating: 0.8° C./s or Higher

If the cooling rate of the cladding metal is below 0.8° C./s,precipitation of sigma phase and/or carbonitrides occurs in the claddingmetal, causing deterioration in the corrosion resistance of the claddingmetal. Therefore, the cooling rate of the cladding metal is set to 0.8°C./s or higher. The cooling rate of the cladding metal may be set to100° C./s or lower.

Cooling Rate of Base Steel Plate After Reheating: 1.0° C./s or Higher

When the cooling rate of the base steel plate is below 1.0° C./s, thehardenability of the base metal is not sufficient, causing deteriorationin strength and/or toughness. Therefore, the cooling rate of the basesteel plate after reheating is set to 1.0° C./s or higher, andpreferably 2.0° C./s or higher. The cooling rate of the base steel platemay be 100° C./s or lower.

The cooling rates can be controlled as described above by setting thecooling method and the cooling conditions in consideration of thethicknesses of the base steel plate and the cladding metal and the formof the clad slab. For example, in the case of using the clad slabillustrated in FIG. 1 , if the rolled clad material is water-cooled fromboth sides, the cooling rate of the base steel plate is higher than thatof the cladding metal. Further, in the case of using a clad slab inwhich one first blank plate and one second blank plate are stacked, thecooling rate can be individually controlled for the base steel plate andfor the cladding metal by changing the water cooling conditions onrespective sides of the rolled clad material. It is noted that thecooling stop temperatures of the cladding metal and the base steel plateafter reheating are both preferably lower than 200° C.

Then, so-called tempering treatment whereby the rolled clad body thuscooled is heated at 700° C. or lower is performed.

Tempering Temperature: 700° C. or Lower

The purpose of tempering treatment is to adjust the strength of the basesteel plate. By performing tempering treatment, the base steel plate canbe adjusted to a desired strength. In addition, tempering treatment isexpected to cause a change in the form of carbides such that thetoughness improves. However, when tempering treatment is performed at atemperature above 700° C., carbides and/or nitrides precipitate in thecladding metal, resulting in deterioration in corrosion resistance.Therefore, the tempering temperature is 700° C. or lower, and preferably650° C. or lower. The tempering temperature may be 200° C. or higher.

Optionally, a known step may be further added before or after each step.For example, in the case of using the clad slab illustrated in FIG. 1 ,the resulting rolled clad body is peeled off with the separating agentapplied between the cladding metals to obtain clad steel plates asfinished products.

The production method disclosed herein can produce a duplex stainlessclad steel plate excellent in all of the corrosion resistance of thecladding metal, the strength and toughness of the base steel plate, andthe bonding or joining strength between the cladding metal and the basemetal.

EXAMPLES

Various first blank plates (blank materials of base steel plates) havingthe chemical compositions listed in Table 1 were prepared. Varioussecond blank plates (blank materials of cladding metals) having thechemical compositions and area fractions of ferrite phase listed inTable 2 and made of duplex stainless steel plates were prepared. Thearea fractions of ferrite phase listed in Table 2 were calculated inaccordance with Expression (3). Various duplex stainless clad steelplates listed in Table 4 were produced by applying various productionconditions listed in Table 3 to the clad slabs obtained by piling firstand second blank plates on top of one another in a layered manner.

TABLE 1 Base metal Chemical composition (mass %) (with the balance beingFe and inevitable impurities) No. C Si Mn P S Al Mo Nb Ti N Cu Cr Ni VNb/N Ceq Remarks  1 0.14 0.24 1.45 0.006 0.005 0.053 0.09 0.024 <0.001 0.0035 6.9 0.40 Example steel  2 0.06 0.32 1.58 0.002 0.008 0.021 0.120.037 0.003 0.0023 16.1  0.35 Example steel  3 0.25 0.13 1.09 0.0090.006 0.049 0.10 0.027 <0.001  0.0064 4.2 0.45 Example steel  4 0.140.34 1.25 0.015 0.005 0.032 0.05 0.022 <0.001  0.0066 3.3 0.36 Examplesteel  5 0.19 0.24 1.29 0.029 0.009 0.047 0.06 0.010 <0.001  0.0021 4.80.42 Example steel  6 0.15 0.29 1.25 0.010 0.002 0.098 0.11 0.040 0.0020.0061 6.6 0.38 Example steel  7 0.16 0.32 1.25 0.006 0.006 0.062 0.080.031 0.004 0.0051 6.1 0.38 Example steel  8 0.17 0.40 1.21 0.001 0.0100.016 0.06 0.019 0.001 0.0010 19.0  0.38 Example steel  9 0.15 0.36 1.220.006 0.008 0.036 0.13 0.037 0.001 0.0098 3.8 0.38 Example steel 10 0.190.12 1.37 0.006 0.003 0.050 0.05 0.036 <0.001  0.0061 0.01 0.01 5.9 0.43Example steel 11 0.13 0.28 1.38 0.019 0.009 0.049 0.13 0.034 <0.001 0.0051 0.02 0.001 6.7 0.39 Example steel 12 0.15 0.31 1.34 0.004 0.0030.024 0.07 0.018 <0.001  0.0060 3.0 0.39 Example steel 13 0.26 0.41 1.050.010 0.008 0.030 0.06 0.025 <0.001  0.0033 7.6 0.45 Comparative steel14 0.12 0.12 1.38 0.017 0.001 0.045 0.09 0.009 <0.001  0.0021 4.3 0.37Comparative steel 15 0.17 0.31 1.29 0.017 0.008 0.041 0.11 0.041 0.0030.0042 9.8 0.41 Comparative steel 16 0.18 0.32 1.36 0.008 0.006 0.0240.08 0.021 0.005 0.0039 5.4 0.42 Comparative steel 17 0.14 0.24 1.270.009 0.005 0.022 0.05 0.015 <0.001  0.0008 18.8  0.36 Comparative steel18 0.19 0.29 1.25 0.004 0.001 0.052 0.09 0.037 <0.001  0.0103 3.6 0.42Comparative steel 19 0.17 0.32 1.14 0.021 0.004 0.031 0.09 0.021 <0.001 0.0072 2.9 0.38 Comparative steel 20 0.11 0.33 1.30 0.001 0.001 0.0530.07 0.038 0.002 0.0067 5.7 0.34 Comparative steel 21 0.22 0.12 1.380.014 0.002 0.053 0.06 0.025 <0.001  0.0055 4.5 0.46 Comparative steelNote: Underlined if outside the range of the disclosure.

TABLE 2 Cladding Chemical composition (mass %) (with the balance beingFe and inevitable impurities) metal No. C Si Mn P S Ni Cr Mo N Cu W CoTi Nb  1 0.023 0.17 1.53 0.032 0.0060 6.48 25.0 3.4 0.15 — — — — —  20.022 0.40 1.59 0.035 0.0040 6.48 24.7 3.9 0.18 0.02 0.01 — 0.01 —  30.023 0.21 1.65 0.015 0.0030 6.95 24.2 3.0 0.20 — — 0.01 — 0.02  4 0.0300.34 1.24 0.031 0.0030 6.09 24.5 3.2 0.18 — — — — —  5 0.022 0.31 1.680.024 0.0020 5.01 25.3 3.7 0.25 — — — — —  6 0.016 0.20 1.50 0.0270.0050 7.99 24.4 3.2 0.18 — — — — —  7 0.014 0.13 1.18 0.033 0.0060 7.3024.1 3.8 0.16 — — — — —  8 0.022 0.42 1.13 0.027 0.0020 6.34 28.0 3.20.24 — — — — —  9 0.019 0.31 1.06 0.019 0.0060 5.59 25.3 2.5 0.24 — — —— — 10 0.022 0.40 1.42 0.027 0.0020 6.07 24.3 4.0 0.17 — — — — — 110.022 0.43 0.98 0.018 0.0070 6.62 24.3 3.4 0.08 — — — — — 12 0.013 0.121.66 0.016 0.0050 5.58 25.6 3.3 0.30 — — — — — 13 0.013 0.35 1.67 0.0330.0030 7.24 24.3 2.5 0.10 — — — — — 14 0.010 0.37 1.54 0.022 0.0050 6.6826.4 3.8 0.25 — — — — — 15 0.010 0.36 1.36 0.017 0.0020 7.40 24.3 2.80.22 — — — — — 16 0.014 0.35 1.22 0.018 0.0020 5.51 25.4 3.5 0.12 — — —— — 17 0.031 0.26 1.31 0.021 0.0070 5.53 24.6 3.9 0.16 — — — — — 180.013 0.22 1.57 0.019 0.0050 4.99 27.5 3.7 0.20 — — — — — 19 0.010 0.161.69 0.026 0.0040 8.02 25.5 3.2 0.25 — — — — — 20 0.023 0.14 0.93 0.0180.0020 7.82 23.9 2.6 0.17 — — — — — 21 0.022 0.13 1.42 0.018 0.0050 5.6229.1 3.2 0.19 — — — — — 22 0.016 0.28 1.44 0.027 0.0060 6.84 24.2 2.40.23 — — — — — 23 0.018 0.43 0.92 0.025 0.0020 6.98 25.6 4.1 0.14 — — —— — 24 0.011 0.28 0.98 0.027 0.0060 5.79 25.3 3.8 0.07 — — — — — 250.016 0.28 1.02 0.031 0.0060 5.53 25.7 3.1 0.31 — — — — — 26 0.018 0.191.31 0.034 0.0020 7.08 24.2 2.5 0.09 — — — — — 27 0.016 0.25 1.52 0.0240.0050 6.87 26.7 3.6 0.28 — — — — — Area fraction Cladding of ferritemetal No. PI Creq Nieq phase (%) Remarks  1 38.6 28.29 10.31 52 Examplesteel  2 40.5 28.82 10.94 50 Example steel  3 37.3 27.20 11.86 39Example steel  4 37.9 27.90 10.74 48 Example steel  5 41.5 29.09 10.9951 Example steel  6 37.8 27.56 12.30 38 Example steel  7 39.2 27.6711.12 45 Example steel  8 42.4 31.54 12.10 55 Example steel  9 37.428.04 11.28 45 Example steel 10 40.2 28.51 10.31 52 Example steel 1136.8 28.04 8.90 58 Example steel 12 41.3 28.71 12.42 41 Example steel 1334.2 27.11 9.73 50 Example steel 14 42.9 30.38 12.36 48 Example steel 1537.1 27.39 12.43 36 Example steel 16 38.9 29.09 8.46 65 Example steel 1740.0 28.48 9.77 55 Comparative steel 18 42.9 31.14 9.66 67 Comparativesteel 19 40.1 28.59 13.70 34 Comparative steel 20 35.2 26.43 12.08 34Comparative steel 21 42.7 32.14 10.29 67 Comparative steel 22 35.8 26.8012.24 35 Comparative steel 23 41.4 29.95 10.47 57 Comparative steel 2439.0 29.13 7.58 70 Comparative steel 25 40.9 28.91 12.67 41 Comparativesteel 26 33.9 26.73 9.48 50 Comparative steel 27 43.1 30.30 13.35 43Comparative steel Note: Underlined if outside the range of thedisclosure.

TABLE 3 Hot rolling Quenching treatment Tempering Heating RollingReheating Cooling rate of Cooling rate of Heating Production temp.reduction temp. cladding metal base metal temp. method No. (° C.) ratio(° C.) (° C./s) (° C./s) (° C.) 1 1200 4.8 1050 1.5 2.8 600 2 1050 3.81050 1.9 3.8 650 3 1250 5.2 1050 2.3 5.2 630 4 1200 2.0 1050 1.2 1.6 5805 1150 7.9 1000 5.1 2.5 500 6 1200 11.0  1100 12.4  19.4  620 7 1200 3.21000 0.8 1.5 610 8 1150 7.6 1050 5.4 1.0 630 9 1100 6.8 1050 6.9 2.6 70010  1040 4.2 1050 2.8 7.1 630 11  1260 5.3 1050 1.1 1.4 650 12  1100 1.91000 0.9 1.8 590 13  1200 3.9  990 1.6 3.9 530 14  1150 11.6  1120 10.7 16.3  580 15  1150 3.1 1050 0.7 1.3 630 16  1100 2.7 1000 7.4 0.9 67017  1200 5.7 1050 10.3  1.9 710 Note: Underlined if outside the range ofthe disclosure.

For the obtained clad steel plates, the following evaluations werecarried out and the results are listed in Table 4.

(1) Measurement of the Amount of Precipitated Cr and the Amount ofPrecipitated Mo

For extraction of precipitates in the cladding metal, electrolyticextraction in a 10 vol % acetylacetone−1 mass % tetramethylammoniumchloride—methanol mixed solution (commonly referred to as 10% AAsolution) was applied (this electrolytic extraction is commonly referredto as SPEED method). The extraction residue collected on the filter byfiltration was dissolved in a mixed acid (with a mixed acid componentratio of sulfuric acid 10 mL:nitric acid 10 mL:perchloric acid 5mL:water 10 mL), and analyzed by inductively coupled plasma (ICP)emission spectroscopy to determine the amount of precipitated Cr and theamount of precipitated Mo.

(2) Evaluation of Corrosion Resistance (Pitting Corrosion Resistance) ofCladding Metal

The corrosion resistance was evaluated in accordance with ASTM G48—TestMethod (E). In the test method, each specimen was immersed in a 6% FeCl₃aqueous solution heated to 20±2° C. for 24 hours, and was judged ashaving good corrosion resistance when pitting corrosion having a depthof 25 μm or more did not occur on the surface of the cladding metalafter the test.

(3) Evaluation of Bonding or Joining Strength

The bonding or joining strength between the cladding metal and the basemetal was evaluated in accordance with JIS G0601—Shear Strength Test.The shear strength test is a method of evaluating the bonding or joiningproperty from the maximum shear strength required for the separation ofthe cladding metal from the base metal in parallel to the bonding orjoining surface. The bonding or joining property was judged as good whenthe shear strength was 200 MPa or more.

(4) Evaluation of Toughness of Base Steel Plate

The toughness of each base steel plate was evaluated by Charpy impacttest. From each base steel plate, a 10 mm×10 mm size V-notch Charpyimpact test specimen prescribed in JIS Z 2242 was sampled and subjectedto Charpy impact test. The toughness was judged as good when the valueof Charpy impact absorbed energy at −20° C. (vE⁻²⁰) was greater than 100J.

(5) Evaluation of Strength of Base Steel Plate

The strength of each base metal was evaluated by tensile test. A JIS 1Atensile test piece was collected from a region where only the base metalis present as a result of removing the cladding metal from the cladsteel plate by machining, and subjected to a tensile test. The temperingtemperature was adjusted such that the tensile strength was about 550MPa.

TABLE 4 Clad Amoung of Amount of steel Base Cladding Productionprecipitated precipitated Shear Tensile plate metal metal method Cr MoPitting stress vE−20 strength No. No. No. No. (mass %) (mass %)corrosion (MPa) (J) (MPa) Remarks 1 1  1 1 0.02 0.00 not occurred 329218 545 Example 2 2  2 2 0.05 0.02 not occurred 234 218 543 Example 3 3 3 3 0.02 0.03 not occurred 346 176 561 Example 4 4  4 4 0.01 0.02 notoccurred 245 181 552 Example 5 5  5 5 0.02 0.00 not occurred 336 241 538Example 6 6  6 6 0.02 0.03 not occurred 318 148 531 Example 7 7  7 70.04 0.01 not occurred 327 173 550 Example 8 8  8 8 1.84 0.03 notoccurred 317 167 530 Example 9 9  9 9 0.03 0.02 not occurred 331 155 554Example 10 10  10 1 0.02 0.02 not occurred 316 265 550 Example 11 11  111 0.04 0.02 not occurred 301 252 536 Example 12 12  12 1 1.24 0.02 notoccurred 325 135 551 Example 13 1 13 1 0.03 0.01 not occurred 312 243558 Example 14 1 14 1 1.09 0.01 not occurred 313 221 555 Example 15 1 151 0.05 0.02 not occurred 309 260 564 Example 16 1 16 1 0.02 0.02 notoccurred 300 234 557 Example 17 4  1 1 0.03 0.00 not occurred 325 176558 Example 18 12   1 1 0.03 0.00 not occurred 315 143 537 Example 19 117 1 2.09 0.01 occurred 328 220 561 Comparative example 20 1 18 1 0.000.00 occurred 304 229 542 Comparative example 21 1 19 1 0.03 0.01occurred 313 232 552 Comparative example 22 1 20 1 0.00 0.01 occurred348 225 555 Comparative example 23 1 21 1 2.16 0.02 occurred 293 243 553Comparative example 24 1 22 1 0.03 0.01 occurred 334 242 570 Comparativeexample 25 1 23 1 0.04 0.59 occurred 355 243 530 Comparative example 261 24 1 0.03 0.02 occurred 309 228 555 Comparative example 27 1 25 1 2.130.02 occurred 308 232 546 Comparative example 28 1 26 1 0.02 0.02occurred 353 234 536 Comparative example 29 1 27 1 2.07 0.63 occurred327 241 536 Comparative example 30 13   1 1 0.02 0.00 not occurred 301 79 537 Comparative example 31 14   1 1 0.03 0.00 not occurred 303  78541 Comparative example 32 15   1 1 0.03 0.00 not occurred 304  61 566Comparative example 33 16   1 1 0.03 0.00 not occurred 318  89 544Comparative example 34 17   1 1 0.01 0.00 not occurred 321  63 550Comparative example 35 18   1 1 0.01 0.00 not occurred 318  68 542Comparative example 36 12   1 1 0.02 0.00 not occurred 324  68 552Comparative example 37 20   1 1 0.03 0.00 not occurred 312  65 568Comparative example 38 21   1 1 0.02 0.00 not occurred 334  49 548Comparative example 39 13  17 10  2.45 0.01 occurred 152  60 531Comparative example 40 13  17 11  2.18 0.02 occurred 321  20 553Comparative example 41 13  17 12  2.23 0.00 occurred 139  11 568Comparative example 42 13  17 13  5.23 0.52 occurred 291  61 541Comparative example 43 13  17 14  2.32 0.03 occurred 299  16 534Comparative example 44 13  17 15  5.41 0.69 occurred 315  11 557Comparative example 45 13  17 16  2.19 0.01 occurred 324  12 541Comparative example 46 13  17 17  2.38 0.01 occurred 336  58 560Comparative example 47 1  1 13  2.08 0.02 occurred 326 206 544Comparative example 48 1  1 15  2.15 0.02 occurred 311 186 557Comparative example 49 1  1 17  2.27 0.01 occurred 308 175 539Comparative example Note: Underlined if outside the range of thedisclosure.

The clad steel plates of Nos. 1 to 18 according to examples exhibitedgood corrosion resistance and toughness. The clad steel plates of Nos.19 to 29 having the chemical compositions of the cladding metals outsidethe range of the present disclosure were inferior in corrosionresistance. The clad steel plates of Nos. 30 to 38 having the chemicalcompositions of the base metals outside the range of the presentdisclosure were inferior in toughness. Further, the clad steel plates ofNos. 39 to 46 having the chemical compositions of the cladding metalsand the chemical compositions of the base metals outside the range ofthe present disclosure were inferior in both corrosion resistance andtoughness. Among these, the clad steel plate of No. 39, which has aheating temperature below the range of the present disclosure, and theclad steel plate of No. 41, in which the rolling reduction ratio isbelow the range of the present disclosure, had a shear strength of lessthan 200 MPa and were inferior in the bonding or joining property. Inthe clad steel plates of Nos. 47 to 49 whose production conditions areoutside the range of the present disclosure, the amount of precipitatedCr was outside the range of the present disclosure, and as a result, thecorrosion resistance was inferior.

REFERENCE SIGNS LIST

-   -   1 first blank plate (blank material of base steel plate)    -   2 second blank plate (blank material of cladding metal)    -   3 separating agent    -   4 spacer    -   5 welded portion    -   10 clad slab

The invention claimed is:
 1. A duplex stainless clad steel plate inwhich a duplex stainless steel plate as a cladding metal is bonded orjoined to one or both surfaces of a base steel plate, wherein the basesteel plate comprises a first chemical composition containing, in mass%, C: 0.06% to 0.25%, Si: 0.05% to 0.50%, Mn: 0.70% to 1.60%, P: 0.030%or less, S: 0.010% or less, Al: 0.005% to 0.100%, Mo: 0.01% to 0.15%,Nb: 0.010% to 0.040%, Ti: less than 0.005%, and N: 0.0010% to 0.0100%,optionally at least one selected from the group consisting of Cu: 0.50%or less, Ni: 0.50% or less, Cr: 0.40% or less, and V: 0.050% or less,with the balance being Fe and inevitable impurities, in a range suchthat Nb/N is 3.0 or more and Ceq represented by the following Expression(1) is 0.35 to 0.45:Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5  (1) where the symbol of each elementindicates the content in mass % of the element, and any element notcontained is calculated as 0, the duplex stainless steel platecomprises: a second chemical composition containing, in mass %, C:0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.050% or less,S: 0.0100% or less, Ni: 5.00% to 8.00%, Cr: more than 24.0% and not morethan 28.0%, Mo: 2.5% to 4.0%, and N: 0.08% to 0.30%, optionally at leastone selected from the group consisting of Cu: 1.50% or less, W: 1.50% orless, Co: 1.50% or less, Ti: 0.25% or less, and Nb: 0.25% or less, withthe balance being Fe and inevitable impurities, in a range such that PIdefined by the following Expression (2) is 34.0 to 43.0:PI=Cr+3.3Mo+16N  (2) where the symbol of each element indicates thecontent in mass % of the element; and a microstructure containing aferrite phase in an area fraction of 35% to 65%, and in themicrostructure, an amount of precipitated Cr is 2.00 mass % or less andan amount of precipitated Mo is 0.50 mass % or less.
 2. A method ofproducing a duplex stainless clad steel plate in which a duplexstainless steel plate as a cladding metal is bonded or joined to one orboth surfaces of a base steel plate, the method comprising: preparing aclad slab by stacking a first blank plate to be the base steel plate anda second blank plate to be the duplex stainless steel plate as thecladding metal in a layered manner, the first blank plate comprising thefirst chemical composition as recited in claim 1, and the second blankplate comprising the second chemical composition as recited in claim 1and a microstructure containing a ferrite phase in an area fraction of35% to 65%; heating the clad slab to 1050° C. to 1250° C.; then hotrolling the clad slab with a rolling reduction ratio of 2.0 or more toobtain a rolled clad body in which the base steel plate and the duplexstainless steel plate are bonded or joined together; allowing the rolledclad body to naturally cool; then reheating the rolled clad body to1000° C. to 1100° C.; then cooling the rolled clad body such that theduplex stainless steel plate is cooled at a cooling rate of 0.8° C./s orhigher and the base steel plate is cooled at a cooling rate of 1.0° C./sor higher; and then tempering the rolled clad body at 700° C. or lowerto produce the duplex stainless clad steel plate of claim 1.